Conventionally, in the technical field which requires high-precision processing, various processing operations are done by setting an object to be processed on a stage which can be aligned at high precision, and controlling the stage. The prior art will be described by exemplifying a projection exposure apparatus used in the manufacture of a semiconductor device or the like.
In the projection exposure apparatus, a reticle stage, which supports a reticle, or a wafer stage, which supports a wafer, must be moved parallel to planes perpendicular to each other along the X- and Y-axes in exposure, and the stage position must be accurately measured and controlled. For this purpose, the projection exposure apparatus uses a laser interferometer as a means for measuring the positions of X and Y strokes on the reticle or wafer stage in the μ order or less.
In general, the reticle or wafer stage slightly rotates within the X- and Y-axis planes (θ-axis direction) (yawing error). The yawing error generated in the reticle or wafer stage also slightly rotates a reticle or wafer set on the stage along the θ-axis, and an error at the periphery cannot be ignored. Therefore, this yawing error must be corrected. For example, a laser interferometer obtains the X-direction positions of two points on the reticle or wafer stage, and a θ-axis displacement is measured from the difference between the positions of the two points and the beam span of the laser interferometer. In this manner, on the reticle or wafer stage, the X-axis position of one point and the Y-axis positions of two points on the stage are generally measured by using the laser interferometer in order to measure X-, Y-, and θ-axis positions.
FIG. 5 is a view showing a measurement principle using a laser interferometer. A bar mirror on an X-Y stage 12 is irradiated with a laser beam from the Y-axis direction, and measurement is done by using the reflected beam. When either of the X and Y strokes is longer, for example, when the Y-axis stroke is longer, as shown in FIG. 5, a bar mirror for measuring an X-axis position inevitably becomes longer along the Y-axis. A long bar mirror makes the apparatus bulky. In addition, a cantilever structure generates deflection and vibrations of the bar mirror itself.
To prevent this, a bar mirror is eliminated from an X-Y stage in the invention disclosed in Japanese Patent Laid-Open No. 5-217837. This X-Y stage will be described with reference to FIG. 6.
In FIG. 6, an X-Y stage 12 comprises a rectangular Y table 14 movable in the Y-axis direction along a pair of rails 13 extending parallel to the Y-axis, and a rectangular X table 16 movable in the X-axis direction along a pair of rails 15 laid parallel to the X-axis on the Y table 14. A wafer W is held on the X table 16.
A laser interferometer is generally constituted by an optical unit which receives a laser beam from a light source, splits it into reference and measurement beams, ensures the optical path of the reference beam, and causes the reference and measurement beams to interfere with each other, a bar mirror for reflecting the measurement beam, a detector for detecting the interference beam, and the like.
A laser head 8 for generating a laser beam, benders for deflecting the optical path of the laser beam, beam splitters located between the benders to split the laser beam, optical units (interferometers) 9a, 9b, and 9c each for splitting the laser beam into reference and measurement beams and ensuring the optical path of the reference beam, and detectors 10a, 10b, and 10c each for detecting the reference and measurement beams are arranged outside the X-Y stage 12. Bar mirrors 11a and 11b for reflecting the measurement beams of laser beams and returning them to the optical units (interferometers) 9a, 9b, and 9c are fixed at the edges of two sides which face the optical units (interferometers) 9a, 9b, and 9c and are perpendicular to each other, thus constituting a laser interferometer.
This laser interferometer measures the positions of the X and Y tables 16 and 14 and the position of the wafer W. A laser beam emitted by the laser head 8 is deflected by the bender and split into two laser beams by the beam splitter. One of the split laser beams is guided to the optical unit (interferometer) 9a where the laser beam is split into reference and measurement beams. The reference beam is repetitively reflected within the interferometer 9a and guided to the detector 10a. The measurement beam emerges from the optical unit (interferometer) 9a, reaches the bar mirror 11a held by the X table 16, and is reflected to return to the optical unit (interferometer) 9a. The measurement beam reaches the bar mirror 11a again, is reflected, and guided to the detector 10a via the optical unit (interferometer) 9a. 
The optical path until the reference beam is incident on the detector 10a is constant regardless of the position of the Y table 14. The optical path until the measurement beam is incident on the detector 10a depends on the Y-axis position of the bar mirror 11a on the X table 16 that reflects the measurement beam, and the measurement beam includes position information of the Y table 14. These optical paths are compared to measure a distance y between the optical unit (interferometer) 9a along the Y-axis and the bar mirror 11a at a point A where the bar mirror 11a held by the X table 16 reflects the measurement beam, and the position of the Y table 14. The other laser beam split by the beam splitter is split into two laser beams by another beam splitter. One of the split laser beams is directly guided to one of the optical units (interferometers) 9b and 9c, whereas the other is deflected in its optical path by another bender and guided to the other optical unit (interferometer). Each of the laser beams guided to the optical units (interferometers) 9b and 9c is split into reference and measurement beams. The measurement beams reciprocate twice between the optical units (interferometers) 9b and 9c and the bar mirror 11b, and the reference beams are repetitively reflected within the respective optical units (interferometers) 9b and 9c. Then, the reference and measurement beams are guided to the detectors 10b and 10c. Distances x1 and x2 between the optical units (interferometers) 9b and 9c and the bar mirror 11b along the X-axis at points B and C where the bar mirror 11b held by the X table 16 reflects the laser beams, and the position of the X table 16 including the two points can be measured from the reference and measurement beams guided to the detectors 10b and 10c. 
The X-axis positions (distances) x1 and x2 of two points on the X table 16 and the Y-axis position (distance) y of one point can provide the position of the X-Y stage 12 and the X-, Y-, and θ-axis positions of the wafer W. In Japanese Patent Laid-Open No. 5-217837, the bar mirrors are arranged on the sides of respective tables in their movement directions on the X-Y stage for measuring the positions of the tables by using the laser interferometer and the bar mirrors for reflecting a laser beam from the laser interferometer. At the same time, the optical units (interferometers) of the laser interferometer are held at side edges facing the bar mirrors of the X table.
In Japanese Patent Laid-Open No. 5-217837, the X-Y stage can be downsized by arranging the bar mirrors outside the stage movable portion. However, the detectors are arranged on the stage movable portion, so optical fibers must be laid out on the stage, complicating the wiring of the moving stage.